Esive tape method of making high temperature polyurethane pressure sensitive adh

ABSTRACT

ORDINARY PRESSURE-SENSITIVE ADHESIVES BASED ON NATURAL OR SYNTHETIC RUBBER ARE MADE HIGH TEMPERATURE RESISTANT BY CURING WITH REACTION-PRODUCTS OF POLYISOCYANATES AND HYDROXYLATED ELASTOMERS.

US. Cl. 117122 P 6 Claims ABSTRACT OF THE DISCLOSURE Ordinarypressure-sensitive adhesives based on natural or synthetic rubber aremade high temperature resistant by curing with reaction-products ofpolyisocyanates and hydroxylated elastomers.

This application is a division of my copending application, Ser. No.822,314, filed May 6, 1969 and now US. Pat. No. 3,671,301 which is acontinuation-in-part of application Ser. No. 762,995, filed Sept. 26,1968 and now US. Pat. No. 3,515,773. I have described pressuresensitiveadhesives made by reacting a prepolymer reaction-product of polyols andpolyisocyanates with hydroxylated elastomers. I have now discovered thatordinary pressure-sensitive adhesives based on natural or syntheticelastomers (rubbers) may be made high temperature resistant by reacting(curing or cross-linking) them with certain prepolymers made by reactionof polyisocyanates with such hydroxylated elastorners.

High temperature resistant pressure-sensitive adhesives have been madein the past by partially curing the natural or synthetic rubber on whichthey are based, by the use of sulfur or phenolic resins. Theseelastomers can also be cured (cross-linked) by isocyanates especially inthe presence of catalysts, but simple addition of isocyanates to theadhesive mass yields a mixture which increases in viscosity so fast asto present serious problems when the mass, for example, is to bedissolved and coated upon a backing to make tape. I have found, however,that this difficulty can be avoided and at the same time greatercohesive strength achieved through use of certain prepolymers made byreacting polyisocyanates with hydroxylated elastomers. These advantagescan be illustrated by adding 1% tolylene diisocyanate (hereinafterdesignated TDI) to one portion of a rubber-based pressuresensitiveadhesive and the same amount of unreacted isocyanate groups (hereinafterdesignated NCO) in the form of a prepolymer reaction-product (made ashereinafter described) of TDI and a hydroxylated polybutadiene polymer,to another portion of the same adhesive. The first mixture gels in amatter of a few hours while the second, after adding a stannous octoatecatalyst and a solvent, can be coated on a backing and dried and curedin an oven even after several weeks. While the first mixture can bedissolved, catalyst added, coated on a backing, the solvent dried outand the mixture cured in an oven, if done immediately after the mix isformed, the cohesive strength of the resulting tape mass is much lowerthan in the case of the second mixture.

The marked improvement in high temperature resistance yielded by mymethods is illustrated by the following examples which may be modifiedand extended within the scope of my invention by those skilled in theart. (All proportions hereinafter are by weight unless otherwise noted.)

United States Patent Patented Sept. 4, 1973 EXAMPLE 1 (A) A normallytacky pressure-sensitive adhesive was made by compounding 250 partscrepe rubber, 150 parts polyterpene resin (Schenectady St. 5115,melting-point 115 C., made by Schenectad chemicals, Inc., Schenectady,N.Y.), 17 parts heat-reactive phenol-formaldehyde resin (Schenectady SP1045, made by Schenectady Chemicals, Inc.), 60 parts petroleumhydrocarbon resin (Piccopale, made by Pennsylvania Industrial ChemicalCorp., Clairton, Pa.), 5 parts 2,6-di-tertiary butyl para cresolantioxidant (Catalin CAO-l, made by Catalin Corporation, New York,N.Y.). The mix was dissolved in toluene to form a solution containing30% solids, knife-coated on an impregnated (saturated) paper backing(made according to the methods of my copending application, Ser. No.297,190) to give a dry coating-weight of 5.5 grams per sq. ft. This wasdried by conveying it through an oven at 250 F. with a dwell-time ofabout 3 minutes.

The resulting tape was a good pressure-sensitive tape with the followingproperties when tested according to the methods of the PressureSensitive Adhesive Tape Council: Quick-Stick 19 oz. (laying on apolished steel surface without pressure and determining the forcerequired to remove a 1-inch wide strip at angle with the surface); 90Peel Adhesion, 24 oz. (laying a 1-inch wide strip on a polished steelsurface, rolling it down with a 4 /2 lb. rubber-covered roller anddetermining the force needed to remove it at 90); 180 Peel Adhesion, 32oz. (same as at 90 except removing it at 180). However, the tape left aheavy residue when stripped from a polished steel plate after beingheated for 30 minutes at 250 F.

(B) A prepolymer was made by reacting parts hydroxylatedbutadiene-acrylonitrile copolymer (85% butadiene, 15% acrylonitrile,hydroxyl number 39) with 12 parts TDI at 215 F. for about 1 hour untilanalysis for free NCO showed that all the hydroxyl groups (hereinafterdesignated OH) of the copolymer had reacted. (This resulted in reactionof substantially all of the more reactive NCO (the 4-position groups ofthe TDI) leaving only the less reactive 2-position NCO which arehindered by their proximity to the methyl groups.) An amount of thisprepolymer corresponding to /2% TDI, based on the weight of crepe rubberin the adhesive (calculating the unreacted NCO as TDI) was added to theadhesive of part A, above, together with an amount of stannousneodecanoate catalyst equal to /2 the weight of prepolymer. Theresulting mix was dissolved to a 40% solution in toluene and coated anddried as in part (A), above. The resulting tape had the same roomtemperature properties as the product of part (A), but it left noresidue when stripped hot from a polished steel plate after 30 minutesat 300 F.

EXAMPLE 2 (A) A pressure-sensitive adhesive consisting of 100 parts SBRrubber (75% butadiene, 25% styrene, Solprene 301, made by PhillipsPetroleum Co., Bartlesville, Okla.), 40 parts ZnO, 60 partspentaerythritol ester of hydrogenated rosin (Pentalyn H, made byHercules Inc., Wilmington, Del.), 20 parts heat-reactivephenol-formaldehyde resin (Schenectady SP 1045) and 10 parts mineraloil, was mixed into 300 parts toluene and 300 parts heptane. Thesolution was coated, dried and tested as in Example 1A and had the sameproperties as the product of 1(A).

(B) A prepolymer, made as in Example 1B, but by reacting 100 parts ofthe hydroxylated styrene-butadiene copolymer, as described later, with13 parts TDI was then added to the adhesive mix of part 2(A) in amountcorresponding to 3% TDI based on the weight of SBR in the adhesive ofpart 2(A), calculating the free NCO in the prepolymer as TDI, plus anamount of stannous octoate catalyst equal to A the weight of prepolymertaken. The 60% solution of this mix in toluene was coated, dried andtested as in Example 1(B). The tape left no residue on a polished steelplate when stripped hot after 17 minutes at 325 F.

EXAMPLE 3 (A) A normally tacky pressure-sensitive adhesive consisting of200 parts synthetic rubber (polyisoprene, 97% cis-1,4 polymer, made byGoodyear Tire and Rubber Co., Akron, Ohio, designated Natsyn 245), 50parts glycerol ester of hydrogenated rosin (Staybelite Ester 10, made byHercules Inc., Wilmington, Del.), 50 parts polyterpene resin(Schenectady St. 5115), 50 parts petroleum hydrocarbon resin(Piccopale), 50 parts phenol-formaldehyde resin (SP 1045), 5 partsmineral oil, 2 parts antioxidant (CAO-l) and 750 parts toluene, wascoated and dried as in Example 1(A) and had the same properties as thetape of Example 1(A).

(B) The prepolymer of Example 1(B) was then added to the adhesive mix ofExample 3(A) in amount corresponding to 1% TDI based on the weight ofpolyisoprene, calculating the free NCO as TDI, plus V2 the weight ofprepolymer used, of stannous octoate catalyst. The mix was dissolved toa solution in toluene and coated and I dried as usual. The product hadthe same properties as the tape of 3(A) at roomtemperature but, unlikeit, left no residue when stripped from a polished steel plate after 30minutes heating at 300 F.

EXAMPLE 4 100 parts of non-curing natural rubber based tape adhesive(similar to the adhesive of Example 1(A) except without thephenol-formaldehyde resin) was mixed with 5 parts of hydroxylatedacrylonitrile-butadiene prepolymer (as in Example 1(B)) and 1 partstannous octoate catalyst. The mix was dissolved to a 30% solution intoluene and coated and dried as in Example 1(A). The resulting tapestripped clean from a polished steel plate after 30 minutes heating at300 F.

EXAMPLE 5 100 parts of the rubber-based adhesive of Example 4 was mixedwith 3 parts of the hydroxylated styrene-butadiene copolymer-isocyanatereaction-product of Example 2(B) and 1 part of stannous octoatecatalyst. The mix was dissolved to a 30% solution in toluene and coatedand dried as in Example 1(A). The resulting tape stripped clean from apolished steel plate after 17 minutes heating at 320 F.

While I have shown the use of TDI in my examples and I prefer thisdiisocyanate because of its relative availability and cost, I may useother polyisocyanates, preferably those having NCO of differingreactivity as in TDI. Some of these are cited in the book entitledPolyurethanes by B. A. Dombrow, published by Reinhold Publishing Corp.,New York, N.Y. (1957) and certain of the isomeric diisocyanates of ethylbenzene, xylene, bitolylene, methyl diphenyl methane, dimethyl diphenylmethane, dianisidine and chlorine-hindered aromatic polyisocyanateswhich I designate as partially sterically-hindered aromaticpolyisocyanates.

In other experiments I have found that the NCO-terminated prepolymer mayalso be made using a hydroxylated polyisoprene elastomer reacted withTDI in a way similar to that used in my examples. In general thewellknown types of normally tacky pressure-sensitive adhesives may bemade high temperature resistant as shown in the examples, whether basedon natural or synthetic elastomers, by combination with NCO-terminatedprepolymers of hydroxylated, liquid polybutadiene,polybutadiene-styrene, polybutadiene-acrylonitrile or polyisoprene. AlsoI may make a normally tacky pressure-sensitive adhesive by 4substituting polybutadiene-acrylonitrile for the SBR of Example 2(A).

The molecular ratio of NCOzOH in reacting TDI with hydroxylatedelastomers can vary from about 1.1 to 2. The prepolymer is normally usedin from 2 to 10 parts per 100 parts of the normally tackypressure-sensitive adhesive as in the A parts of the examples. Thepolyurethane catalyst may be stannous octoate or neodecanoate or thestannous salt of any aliphatic carboxylic acid with from 1 to 20 carbonatoms. I may also use tertiary amine catalysts, lead naphthenate or thecatalysts of US. 3,392,128 or 3,397,158 such as dibutyl tin laurate andother stannic salts or mixtures of catalysts. But stannous octoate andneodecanoate are very active and when less active catalysts are used myhigh temperature resistant tape may have to age a week at roomtemperature before the properties described are fully developed. Curingtimes and temperatures are usually as shown hereinbefore but I may usefinal heating up to 350 F. for a few seconds to effect quick final cure.Catalysts are used in amounts from 25 to 100% of the weight of theprepolymer. The prepolymer may be used in amounts from about A to 3% ofthe TDI equivalent of the unreacted NCO left in the prepolymer after itsformation, based on the weight of elastomer (rubber) in the adhesive towhich the prepolymer is added. In making the prepolymer, any temperaturefrom about 215 F. down to room temperature may be used as long asanalysis shows all OH groups of the hydroxylated elastomer, andpreferably all of the more reactive NCO are reacted. At roomtemperature, the prepolymer reaction takes overnight.

The hydroxylated elastomers I use are all liquids described as follows(1) styrene-butadiene copolymer, 15 to 20% styrene (preferably 20%),hydroxyl number 42, 60% trans, 20% cis (1-4), 20% vinyl (1-2), viscosity295 poises at 30 C., 7.6 lbs. per gallon, iodine number 335 (2)acrylonitrile-butadiene copolymer, 10 to 20% acrylonitrile (preferably15%), hydroxyl number 39, 60% trans, 20% cis (14), 20% vinyl (l-2),viscosity 500 poises at 30 C., 7.7 lbs. per gallon, iodine No. 345 (3)polybutadiene, hydroxyl number 45, viscosity 200 poises at 30 C., 7.5lbs. per gallon, iodine number 355, 60% trans, 20% cis (L4), 20% vinyl(14). The hydroxyl numbers may vary from 20 to 60, although I prefer thehydroxyl numbers shown in the exam les for the respective elastomers.The proportions of trans, cis and vinyl may vary from 57 to 63, 19 to 21and 19 to 21 respectively. The viscosities may vary plus or minus 5% andthe iodine numbers similarly plus or minus 5%.

The TDI used in my examples was a commercial product was 2,4 and 20% 2,6isomers and I generally treat it in my calculations as if it were allthe 2,4 isomer. The tape backing used in my examples is an impregnated(saturated) paper but other types of backings can be used and, ifimpregnated, can be made using butyl or other synthetic rubber as ingeneral use. Coating weights usually run 4 to 7 g./sq. ft.

Other well-known antioxidants can be used such asphenyl-alpha-naphthalamine or the condensation product of butyraldehydeand aniline, but the one used in my examples is cheap and non-staining.I prefer to use tackifying resins of low active hydrogen content andplasticizers of low acid number, but reaction of free NCO in theprepolymer is so rapid in reacting with the active hydrogen in therubber (elastomer) that this is not imperative.

Although I prefer to use a polyisocyanate with NCO groups of differingreactivity as enumerated, I may still use aromatic polyisocyanates inwhich the NCO groups are of essentially equal reactivity, including 2,6and 3,5 TDI. The polisocyanates I use, irrespective of Whether their NCOgroups are of equal or differing reactivity, I designate merely asaromatic polyisocyanates. When I use a polyisocyanate which I havedescribed as a partially sterically-hindered polyisocyanate (which meansa polyisocyanate containing NCO groups of differing reactivity) ormerely as aromatic polyisocyanates, I still calculate the proportion ofprepolymer to rubber of the adhesive in terms of the TDI equivalent ofthe unreacted NCO.

My adhesive mass containing prepolymer may be dissolved for coatingpurposes in any inert solvent of volatility similar to that of tolueneor heptane.

Having thus described my invention, what I claim is:

1. The method of making a high temperature resistant pressure-sensitivetape which comprises (1) adding to a normally tacky pressure-sensitiveadhesive mass based on a rubber selected from the class consisting ofnatural rubber, butadiene-styrene rubber, -butadiene-acrylonitrilerubber and polyisoprene rubber, a prepolymer made by reacting at leastone of the hydroxylated synthetic elastorners selected from the classconsisting of polybutadienestyrene, polybutadiene-acrylonitrile,polybutadiene and polyisoprene with at least one aromatic polyisocyanatein the proportion of NCO:OH of 1.1:1 to 2:1, in amount of prepolymerfrom A to 3% by weight based on the weight of rubber in said adhesivemass, of the toluene diisocyanate equivalent of the free isocyanategroups in said prepolymer; (2) dissolving the product of part (1) in avolatile, inert solvent to make a solution of solids content between 30and 60% by weight; (3) adding to the product of part (2) at least onecatalyst in amount from about 25% to 100% of the weight of saidprepolymer, selected from the class consisting of stannous and stannicsalts of aliphatic carboxylic acids of from 1 to 20 carbon atoms permolecule, dibutyl dilaurate, tertiary amines and lead naphthenate; (4)coating the product of part (3) on a flexible backing material; and (5)heating the product of part (4) to a temperature and for a timesufiicient to remove the said solvent and substantially completely reactthe isocyanate groups in said prepolymer. 2. The method of claim 1wherein the said catalyst selected in part (3) is stannous octoate.

3. The method of claim 1 wherein the said catalyst selected in part (3)is stannous neodecanoate.

4. The method of claim 1 wherein the said aromatic polyisocyanate istolylene diisocyanate containing a major proportion of the 2,4 isomerand the said unreacted isocyanate groups are essentially those in the2-position of said tolylene diisocyanate.

5. The method of claim 1 wherein the said aromatic polyisocyanate is apartially sterically-hindered aromatic polyisocyanate.

6. The method of claim 5 wherein the said free isocyanate groups of thesaid prepolymer are the less reactive isocyanate groups of saidpartially sterically-hindered aromatic polyisocyanate.

References Cited UNITED STATES PATENTS 3,246,049 4/1966 Webber 117-122 X3,427,366 2/1969 Verdol 260-859 3,085,903 4/1963 Bemmels et al ll7-122 XWILLIAM D. MARTIN, Primary Examiner B. D. PIANALTO, Assistant ExaminerUS. Cl. X.R.

l17--122 PB, 161 KP, 161 A, 161 UT; 260-77.5 CR, 858, 859 R

